Ambient lighting in automotive applications is nowadays achieved by providing one or more-colour-light emitting diodes (LEDs) located in various positions in the car. Each of the LEDs is individually controlled in intensity and colour by an integrated circuit (IC) via a communication interface. The LEDs must be calibrated by means of the IC before use to a given target value. A diagnosis of the connected one or more colour LED must be performed during operation.
In today's applications the ambient lighting functionality is assembled on a single Printed Circuit Board (PCB), which requires a certain amount of space. For some applications it requires too big a space, especially if LEDs (and their controlling units) are very close to one another. Hence, there is a constant need to reduce the space occupied by the one or more colour-light emitting diodes significantly by further integration.
It is expected that the number of light emitting diodes used in cars will drastically increase in the coming years. Prognoses of more than 200 interior light sources have already been given for the near future. Given this evolution it is evident that there is a main concern to keep the cost of such a light source as low as possible. The cost issue is valid for the smart LED device, the controlling IC as well as for the used package.
It is known that LEDs as a light source produce a significant heat. In classic approaches a heat sink and a good thermal contact are applied to these applications. It is obvious, that a certain space for the heat sink is needed. This might be challenging in case of space limitations. Material like copper or aluminium for the heat sink is needed. Another way to solve the heat problem is the use of LEDs in a bigger matrix. The LEDs are usually put in a big matrix configuration over a given area, so that heat is not generated on a single point for the full power, but distributed over the area. This leads to a lower junction temperature of the LEDs and a longer life time. The use of so called micro-LEDs, i.e. very small LEDs, has been suggested for that purpose. Space saving has also been a motivation to apply semiconductor technology for interconnecting the LEDs, as bonding the micro-LEDs would take too much space.
Micro-LED technology comes from display applications, which, however, are covering a different functional scope. Therefore the packaging used in such display and screen applications is completely different from what is needed in automotive applications.
In today's automotive ambient light applications mostly three or four LEDs (Red, Green, Blue and White Colour) are packed together in one specialised package that contains a heat sink and/or a very good thermal contact to an external heat sink. The package is constructed with materials, which resist the relatively high temperature produced/caused by the LEDs.
FIG. 1 illustrates a LED device (16) as known in the art. On a glass or any other transparent substrate (5) Light Emitting Diodes (e.g. microLEDs) (1) of only one or more colours, e.g. red (1.1), blue (1.2) and green (1.3), are mounted e.g. by means of a microtransfer technology. On the same substrate one or more metallization layers (e.g. Aluminium or Copper) and insulation layers (e.g. Silicon Oxide, Silicon Nitride or Polyimid) are put as well, e.g. with a semiconductor technology (e.g. layer deposition, layer structuring). These one or more metallization layers connect the LEDs towards connection pads (4). In FIG. 1 only one interconnection layer with 4 wiring traces (2.1, 2.2, 2.3, 2.4) is shown. In case of a stacked assembly an additional protection layer (6) might cover the LEDs and the wirering. The LED device as described is well known e.g. for use in display applications.
The use of a multitude of microLEDs instead of classic LEDs is advantageous in that it reduces the junction temperature significantly from ca. 900° C. to 150° C., because the heat is distributed over a given surface. An LED device according to FIG. 1 thus generates less self-heating compared to conventional LED devices because it is composed of many small LED cells spreading over a large area on a substrate, where the heat can spread out easily.
WO2008/002088 discloses a lead frame having heat sink supporting parts, as well as a light emitting diode package in which the lead frame is employed. The lead frame includes an outer frame surrounding a predetermined region. The heat sink supporting parts extend inward to face each other from the outer frame. Each of the supporting parts has an end portion coupled to a heat sink. Further, lead terminals extend inward to face each other from the outer frame. The lead terminals are spaced apart from the supporting parts. Accordingly, a package main body can be formed by an insert molding technique after the heat sink is coupled to the end portions of the supporting parts, and the heat sink and the lead terminals can be easily aligned.
There is a need for finding a suitable package for such a LED device comprising a large number of light emitting sources and steered by means of an integrated circuit controller, given rather stringent space limitations.